1.1 Field of the Invention
The present invention relates to magnetic lens apparatus comprising magnetic monopoles or magnetic dipoles useful for focusing a beam of charged particles, such as electrons, onto a target in front of the lens. More particularly, the invention concerns magnetic lenses which may be positioned below the specimen in a scanning electron microscope, which have magnetic field strengths on the order of between about 0.5 and 10 Tesla. Disclosed are lenses optimized for high-resolution focusing of an electron beam having an accelerating voltage of between about 10 and about 50,000 V. In a preferred embodiment, the lens apparatus disclosed herein provide the sole focusing lens for a low-voltage (10 to 10,000 V) high-resolution scanning electron microscope. Alternative embodiments employing superconducting coil technologies for producing a magnetic field in such lens apparatus are also disclosed as are methods for their use in variable-probe lithographic etching processes.
1.2 Description of the Related Art
1.2.1 Electron Microscopy
The scanning electron microscope (SEM) is the instrument of choice for the investigation of irregular surfaces both in biology materials sciences and the semiconductor industry. The SEM forms an image by focusing an electron probe onto the surface of the specimen and the image contrast is formed using the secondary electrons or the high energy backscattered electrons which are generated at or near to the surface. Since the depth of focus may be quite large there is no penalty to be paid for deep indentations or sharp projections and as a result the SEM has been used very effectively for the study of such diverse specimens as the surface of cell membranes or semiconductor circuits.
The normal form of the SEM is an instrument which uses electrons of around 30 kV to form the probe. The reason for this choice of voltages is that it is in this range where the electron probe can take on dimensions of relevance to the investigations of these various specimens. Probes can be formed with dimensions of the order of about 0.5 nm, producing effective resolutions in the range of about 0.7 nm.
It should also be noted that these very high resolving powers have been obtained by inserting the specimen inside the magnetic field of the lens which forms the probe and for best operation the specimen is placed at the point of maximum magnetic field at the center of the lens. The maximum magnetic field which is attainable with conventional materials is about 2 or 2.5 T and it is this which limits the focal length which can be obtained with these lenses and in turn it is the focal length which determines the aberration coefficients of the lens.
1.2.2 Unsuitability of High-Voltage Electrons for Delicate Specimens
Unfortunately, there is a penalty to be paid for using high voltage electrons, particularly when biological or other fragile specimens are examined. Such electrons penetrate deeply into specimens of these types to a distance on the order of a micron or two and yet the secondary electrons which form the image are generated only within the top 2 nm or so of the specimen. The consequence of this is that although the probe is small and high resolution can be obtained, the majority of the electrons penetrate deeply into the specimen and cause substantial damage. In the case of biological materials this can cause significant mass loss and even collapse of the specimen. In the case of semiconductors it has the consequence that the area that is being investigated can no longer be used in a working circuit.
1.2.3 Limitations of Conventional Magnetic Lenses
Focusing a beam of charged particles, e.g., electrons, by causing it to pass axially through a magnetic field of symmetrical distribution produced by a current-carrying coil positioned around the beam is well known in the mechanical arts. The focusing of charged particle beams, and particularly electron beams, is of paramount important in the illuminating systems of scanning- and transmission-type electron microscopes, and in electron probe X-ray micro-analysis apparatus.
In each of these apparatus, and particularly in the case of scanning electron microscopes (SEMs) and micro-analysis devices, a critical aspect of the lens is its ability to focus the electron beam on a small target area. Such a requirement has demanded that the resolutions of SEM lenses be of the highest possible order, and that chromatic and spherical aberrations of the lens be of the lowest possible order.
Conventional magnetic lenses which are used in these forms of apparatus are positioned about the path of the beam and may occupy a considerable distance along the beam. Such lenses so positioned can impose undesirable structural and design limitations on the apparatus in which they are used. One such example is illustrated in U.S. Pat. No. 4,419,581 to Nakagawa (specifically incorporated herein by reference). While an improvement over earlier designs, the size, shape, and above-stage configuration of the lens apparatus made it unsuitable for analysis of large specimens such as semiconductor wafers or low-voltage analysis of biological specimens.
Subsequent improvements in electron optic lens systems have been marginal. For example, the magnetic condenser lens system described in U.S. Pat. No. 5,241,176 to Yonezawa (specifically incorporated herein by reference), while employing conical lenses and tapered pole pieces to reduce the distance between the objective lens and the sample, still was an above-stage lens, and the improvement in resolving power over conventional SEM lenses was only about 50%.
Another limitation of these prior art lenses, was that all were closely positioned around the electron beam so that a very powerful pumping system was required to maintain the desired vacuum conditions within the constricted passageways (Mulvey, 1975).
1.2.4 Conventional Above-Specimen Lens Designs Limit SEM
Cylindrically symmetric magnetic fields are in general use for focusing beams of charged particles. These fields are usually near Gaussian in shape and the focus can be inside or outside the strong field region, but virtually any shape will suffice since the focusing only depends upon the square of the field strength. The advantages and disadvantages of any particular choice of field distribution are reflected in three significant electron optical parameters: The focal length f, the coefficient of spherical aberration C.sub.s and the coefficient of chromatic aberration C.sub.c. It is these three numbers which determine the size of the focused probe.
In practice, there are many other considerations which determine the final choice of field shape, among the most important being the size and position of the pole pieces of the lens itself since they may permit or obstruct access to the region around the focus and this may become a serious issue in the case of large specimens and a substantial degree of specimen tilt. For this reason, it is desirable to create alternative lens designs which would provide superior improvements over conventional lens designs, and substantially further the fields of electron microscopy and related applications such as the use of lens apparatus in lithographic processes.
In conventional SEMs, the focusing lens is located between the source and the specimen (the focus) or alternatively the focus occurs inside the lens (e.g., see U.S. Pat. No. 5,241,176 to Yonezawa). The former allows for a substantial working distance at the expense of large values of the three parameters while the latter offers the smallest parameter values at the expense of strict constraints on specimen size.
Although magnetic lens systems have been described which have a conducting coil positioned behind the target position with respect to the beam of particles in order to overcome such mechanical difficulties, these lenses were never commercially available for use in SEMs. One such system was described in U.S. Pat. No. 3,870,891 to Mulvey (specifically incorporated herein by reference). While the configuration of the Mulvey lens provided an improvement over the conventional magnetic lens systems of contemporary SEMs, it was not considered for high-resolution focusing of low-voltage particle beams.
1.3 Deficiencies in the Prior Art
A major limitation of conventional lenses is that they typically have two sets of pole pieces, one above the specimen and one below. The specimen must be placed in this confined space and therefore the separation between the two sets of pole pieces must be of the order of several millimeters. This mechanical limitation significantly restricts the focal length to something of the order of a millimeter or two.
In the conventional design of magnetic lenses it is impossible to achieve the focal length required for very high resolutions and the reason for that is a simple mechanical one. The specimen must be placed in the magnetic field and it must be held in some form of mechanical device which allows for translation and perhaps rotation and tilt of the specimen. These mechanical requirements place a restriction on the smallest dimensions which the lens could have and it is virtually impossible to reduce the size of these mechanical components any further. It is for these and other reasons that there currently is a need in the art for lens apparatus which are capable of high-resolution focusing of a beam of charged particles, and in particular, a low-voltage beam of electrons for examination of specimens in an SEM. The uses for such lens apparatus would not be restricted to use in electron microscopy, but also provide significant improvement in the quality and resolution of etching probes in lithographic processes.
The creation of such a lens apparatus, and in particular lenses employing superconducting coils would providing substantial improvement in the resolving power of contemporary magnetic lenses, and also permit for the first time, high-resolution focusing of low-voltage particle beams. The use of magnetic monopoles and magnetic dipoles in combination with superconducting coils would represent a breakthrough in magnetic lens technology and greatly advance the fields of scanning electron microscopy (particularly in the analysis of biological and other delicate specimens such as semiconductor wafers and related computer devices for which low-voltage beam analyses are tantamount) and lithography (particularly in the fabrication of multiple etching probes having varying probe diameters.